There is considerable demand for optical, physical medium dependent (PMD) networks capable of an increased speed or bandwidth of data transmission. Currently, high speed optical networks may provide a speed or bandwidth in a range of 100 gigabits per second (100 Gb/s). There is demand, however, for bandwidth in the range of 400 Gb/s and even 1 terabit per second (1 Tb/s). Such increased bandwidth enables all forms of data sharing and processing to function far more quickly and with fewer delays between each associated node on the network and more nodes to be accommodated on the same optical network without modification of the underlying equipment.
Bandwidths above 100 Gb/s, however, are difficult to achieve using currently known technologies.
One alternative used is wavelength division multiplexed (WDM) technology. WDM systems use a separate wavelength for each separate signal—which requires a plurality of light sources and a wavelength division multiplexer for multiplexing and de-multiplexing the different wavelength signals or channels.
Another alternative used is quadrature amplitude modulation (QAM) technology. QAM systems use changing or modulating amplitudes of two carrier waves with the amplitude-shift keying (ASK) digital modulation scheme. The two carrier signals, usually sinusoids, are out of phase with each other by 90 degrees and are thus called “quadrature” carriers or components—which is reflected in the name of the system.
A third alternative used is intensity modulation (IM) or intensity modulation and direction detection (IM/DD) technology. IM and IM/DD are a form of modulation in which the optical power output of a source is varied in accordance with some characteristic of the modulating signal. The envelope of the modulated optical signal is an analog of the modulating signal in the sense that the instantaneous power of the envelope is an analog of the characteristic of interest in the modulating signal. Recovery of the modulating signal is usually by direction detection.
A fourth alternative is discrete multitone modulation (DMT) technology, which uses fast Fourier transform operations at both transmitter and receiver ends.
A fifth alternative is polarization-division multiplexing (PDM) or dual polarization, coherent detection and digital signal processing technology. PDM systems may be used together with phase modulation or QAM, allowing transmission speeds of 100 Gbit/s or more over a single wavelength.
One issue with the use of PDM over fiber-optic transmission systems are the drifts in polarization state that may occur due to physical changes in the fiber and other factors, Over a long-distance system, these drifts may accumulate, resulting in irregular rotation of the polarized light's Jones vector over the entire Poincaré sphere. Polarization mode dispersion, polarization-dependent loss, and cross-polarization modulation are other phenomena that may cause problems in PDM systems.
Thus, there exists a need for an optical transmitter and method of transmission that permits the synchronization of multiple, polarized optical signals and combination of these signals in a form that preserves the differences in their polarization vectors.